-
Hydrophilic Modified clay Nanocomposites: Effect of clay on
Thermal and Vibrational Properties
A. Kishore1, D. B. Venkatesh1, M. Ashok Kumar2,*, A.
Ramesh2,
K. Nikil Murthy3, N. Karthikeyan4
1Vardhaman College of Engineering, Kacharam - Village,
Shamshabad,
Ranagareddy - Dist 501218, India
2Department of Mechanical Engineering GATES Institute of
Technology, Gooty,
Anantapur - 515401, Andhra Pradesh, India *Telephone: (+91)
9441115859
3Department of Mechanical Engineering, Brilliant Grammar School
Educational Society’s Group of
Institutions Engineering and Phamacy, Abdullapur Village,
Hyderabad, Ranga Reddy District, India
4Kasireddy
Narayan Reddy, College of Engineering and Research, Abdullapur
Village,
Ranga Reddy District, India
*E-mail address: [email protected]
ABSTRACT
Epoxy (LY-556/HY-951) system filled with modified clay (MC) was
synthesized by using
mechanical shear mixing with the addition of hardener as
tri-ethylene-tetra-amine (TETA). The effect
of the fumed silica can be negated by the application of a shear
force (e.g. mixing, brushing, spraying
etc), allowing the liquid to flow, level out and permit the
escape of entrapped air. The reinforcement
effects of MC in the epoxy polymer on thermal, mechanical and
vibration properties were studied.
Curing study shows that the addition MC does not show any effect
in the curing behavior of epoxy
polymer. Thermogravimetry analysis (TGA) shows enhanced thermal
stability for epoxy with MC
fillers. The epoxy with MC fillers shows considerable
improvement on tensile and impact properties
over pure epoxy polymer. SEM studies shows that addition of clay
significantly turns the epoxy
system from brittle to ductile nature was played instrumental in
scaling performance. The
improvement in tensile and impact properties of nanocomposites
is supported with the fracture surface
studies. Epoxy with MC fillers shows enhanced vibration
characteristics than that of the pure epoxy
polymer. FTIR studies indicated the formation of C-H bonds on
the surface of the nanocomposites.
Keywords: Hydrophilic nanoclay; Epoxy Nanocomposites; Mechanical
properties
1. INTRODUCTION
The epoxy polymers used as adhesives and as the matrices of
composite materials are
amorphous and highly crosslinked thermosetting materials. These
chemical structures result in
many useful properties such as a high modulus and failure
strength, low creep and good
performance at elevated temperatures. However, this chemical
structure also leads to one
highly undesirable property in that they are relatively brittle
materials, with a poor resistance
International Letters of Chemistry, Physics and Astronomy
Online: 2014-02-06ISSN: 2299-3843, Vol. 27, pp
73-86doi:10.18052/www.scipress.com/ILCPA.27.73CC BY 4.0. Published
by SciPress Ltd, Switzerland, 2014
This paper is an open access paper published under the terms and
conditions of the Creative Commons Attribution license (CC
BY)(https://creativecommons.org/licenses/by/4.0)
https://doi.org/10.18052/www.scipress.com/ILCPA.27.73
-
to crack initiation and growth. The authors have previously
shown that nanocomposites may
be successfully manufactured using 20 nm diameter silica
nanoparticles, and that these
particles increase the toughness of the material. Siegel et al.
[1] obtained an increase of 15 %
of the strain to failure filling an epoxy resin with 10 wt. % of
nanometric TiO2 particles.
Evora et al. [2] found that adding only 1 vol. % of TiO2
nanoparticles within unsaturated
polyester resin increased the fracture toughness of about 57 %
was due to the uniform and
fine dispersion of the filler within the resin at low volume
contents. More significant
enhancements in fracture toughness (almost 100 % at 4.5 vol. %
of Al2O3 nanoparticles in
unsaturated polyester) were achieved improving the
particle-matrix adhesion through a silane
surface treatment [3]. Wetzel et al. [4] studied the effects of
nano (alumina) and micro-
spherical (calcium silicate) particle addition to epoxy resin
and found increases in flexural
modulus (up to 35 %), strength (up to 20 %) and Charpy impact
energy (up to 35 %). In a
following, interesting work [5], neat epoxy reinforced with
Al2O3 nanoparticles at different
volume contents was investigated. The 10 vol. % epoxy/ Al2O3
nanocomposite exhibited
significant improvements in flexural modulus (around 40 %),
strength (15 %) and fracture
toughness (120 %). Furthermore, the crack propagation threshold
and resistance turned out to
be improved dramatically, with the crack propagation rates for
nanocomposites being orders
of magnitude slower than neat resin for the same range of SIF.
Adebahr et al. [6] proposed a
novel route to prepare nanocomposites consisting of
monodispersed SiO2 nanoparticles and
reactive resin. The addition of 23 wt. % of particles subjected
to thermal anhydride curing
induced a 66 % increase in KIC, while UV curing led to an
improvement of 82 % at 50 wt. %.
Lin et al. [7] reported that tensile and impact strength of
titanium dioxide and montmorillonite
filled epoxy resin reached a maximum for a filler content of 5-8
vol. % and decreases at
higher filler contents, sometimes even below the neat resin
values. Ragosta et al. [8] improved
the mechanical properties of epoxy resin adding 10 wt. % of
silica particles with a diameter of
10-15 nm. The normalized elastic modulus reached the value of
1.5, while the normalized
yield strength increased up to 1.3. The addition of silica
raised the fracture energy of the
epoxy matrix by a factor of about 4, whereas the increase of KIC
was twofold. Zheng et al. [9]
found that the addition of 3 wt. % of silica nanoparticles
within epoxy matrix leads to an
increase in tensile strength of 115 %, while the impact strength
increases by 56 %. In the
literature, the toughening effect due to the addition of
particles to polymers has been studied
for a long time [10-12]. Different toughening mechanisms have
been mentioned, such as the
localized inelastic matrix deformation and void nucleation,
particle debonding, crack
deflection, crack pinning, crack tip blunting, particle
deformation or breaking at the crack tip.
However, it is still an open question which is the effective
mechanisms responsible for
toughening on nanocomposites [13]. Furthermore, experimental
techniques and descriptive
models are based on macro-mechanical concepts. Thus, their
application to nanocomposites is
not straightforward and indeed questionable. Particle-matrix
debonding and localized
deformations in the process zone ahead of the crack tip are
probably responsible of the
considerable toughening effect brought by nanomodification.
Recent experimental
investigations by Johnsen et al. on silica nanoparticle
reinforced epoxy polymers confirm
these assumptions [14]. Because of the very high specific
surface area, even very low filler
contents can significantly contribute to matrix reinforcement.
Especially interface related
effects, such as debonding mechanisms and void nucleation could
play a significant role even
at low volume contents. Although classical mechanical theories
concerning particle
toughening sometimes even predict a decrease of toughening
contribution with decreasing
particle size, the increasing amount of interfacial area and
absolute number of particles in the
process zone can be reasons for the experimentally observed
increases in KIC [15]. Xie et al.
74 ILCPA Volume 27
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[16] reported the improvement of the mechanical properties of
PVC with the addition of
CaCO3. At 5 vol. %, optimal performances were achieved in
Young’s modulus, tensile yield
strength, and strain to failure and Charpy impact energy. The
filler enabled ductile fracture
caused by elevated triaxial stresses at the neck region and
consequently debonding at the
particle-matrix interface. Increasing the load, the ligaments
between the voids were stretched
increasing the energy consumption. The unmodified MMT clay
addition leads to the
conventional composites. The exfoliated structure possesses
superior properties among the
three existing structures. The high surface contact area of
matrix polymer to nanolayers,
uniform distribution of nano layers, etc. of exfoliated
structure enhance the properties than
other two structures [17-20]. Thermoset epoxy-clay
nanocomposites (ECN) are studied under
different curing conditions, synthetic routes, organoclays, etc.
The report suggests that good
exfoliation can be achieved in ECN when amine based curing
agents are used [21-24].
The main objective of the researcher is to attempt a lighter
material which promises to
have high performance applications, as nanocomposites form a
good platform for generating
lighter materials. Although several types of amine curing agents
are available in the literature, limited reports are available on
TETA curing agent [25-31]. The TETA curing agent is widely
used for making glass fibre reinforced epoxy composites. The
main purpose of nanoclay filler
is to increase the matrix properties of fibre reinforced polymer
composites. The addition of
nanoclay in epoxy resin with addition of TETA curing agent is an
important phenomenon to
consider as large amount of glass fibre reinforced composites
are used in several applications.
In this work, the effect of modified MMT clay when dispersed
into the epoxy polymer matrix,
under TETA curing is studied. The curing behaviour, structure,
tensile, thermal and
vibrational, SEM and FTIR properties are studied as a function
of clay concentrations.
2. EXPERIMENTAL DETAILS
2. 1. Materials
The matrix material used in this present study is a commercially
available epoxy resin
(Aradur LY-556) and hardener (TETA, Araldite HY-951) supplied by
Huntsman,
Switzerland. nano filler was used in this study Montmorillonite
clay (Product No: 682659;
Brand: Aldrich, Product name: Nanoclay, hydrophilic bentonite;
Formula: H2Al2O6Si;
Molecular weight: 180.1 g/mol; Appearance (Color): Conforms to
Requirements Light Tan to
Brown; Appearance (Form): Powder; Loss on drying: ≤18.0 %;
Density: 600-1100 kg/m3;
Bulk density: Avg. particle size: ≤25 mocron) supplied by
Sigma-Aldrich Chemicals Pvt.
Limited, Bangalore, India. The surface of the clay was
chemically modified with coupling
agent as 3-aminopropyltrimethoxysilane. The surface modification
processes as well as the
manufacturing parameters are reported elsewhere [18].
2. 2. Nanocomposite fabrication
Initially epoxy resin is heated at 80 °C for 1 h. Clay was kept
in the oven for about half
an hour at 50 °C to keep the moisture at bay. The clay is then
gently added in to the resin
bath. Mixing of clay and epoxy is carried out by mechanical
shear mixer. The mixer rotates at
1000 rpm and mixing is carried out for 2 h. After uniform mixing
of clay and resin, TETA
hardener is added in to the resin/clay solution. The resin to
hardener ratio is maintained 10:1,
and then casted in the mold. The mold is kept at 80 °C for 4 h
until complete polymerization
occurs. Hand-lay up technique was used to impregnate the
composite structures. The
nanocomposite specimens synthesized by this method are tested
for various characterizations.
International Letters of Chemistry, Physics and Astronomy Vol.
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2. 3. Characterization
Curing characteristics of epoxy and epoxy filled MC series was
studied using DSC. The
sample (consisting of resin, clay and hardener prior to curing)
of 5 mg is taken in an alumina
crucible and heat is applied at the rate of 10 °C min. Modal
analysis is performed to calculate
natural frequency and damping factor ‘n’. Damping factor is
calculated using impulse
hammer technique (IHT) and logarithmic decrement method (LDM).
Specimens of 250 mm x
25 mm x 4 mm is prepared, in which one end of beam is clamped
and other end is attached to
accelerometer to obtain vibration modes. Natural frequency is
determined by impulse loading
at free end of the bean using impulse exicitation (Rion PH 7117,
modally tuned hammer). The
signal received from accelerometer is displayed in Fast Fourier
Transform (FFT), in which
natural frequency is noted down for various modes. Damping
factor ‘n’ using IHT is
determined using half power bandwidth method.
The expression for damping factor by half power width technique
is given by (
n 2/ ), where is bandwidth at half-power points of resonant peak
for the nth
mode and n is resonant frequency. The half power points are
found at 2/1 of maximum
peak value. In the LDM, sine wave signal is supplied to drive
the modal exciter to excite the
cantilever beam specimen. During the natural frequency mode,
amplitude increases to a large
extent, once the resonance is achieved. At this point, the
excitation signal is disconnected
freely and a typical free decay curve is obtained. From this
decay curve, two experimental
amplitude data points are collected namely x1 and x n+1, and the
damping factor n is calculated
by using the expression
1
1ln1
1
nx
x
n
22
2
A
where , the damping factor, n + 1, the number of cycles, is the
logarithmic decrement, x1
and x n+1 are the two displacement values at the time intervals
t1 and t2, respectively. A Jeol
JSM 840A Japan scanning electron microscope (SEM) was used to
study the morphology of
fractured surfaces of silica/clay nanocomposite samples at
different magnifications. The
fractured surfaces of tensile test specimens carried out using
SEM. Samples were gold-coated
initially subjecting it to SEM analysis.
The scanning electron micrograms of different cross-sections of
the nanocomposite
samples of pure epoxy, filled with fumed silica and clay
nanocomposites, are studied. Tensile
strength was studied using an universal testing machine (UTM)
supplied by Instron
Corporation; 3369, series-9 automated testing machine was used
with a crosshead speed of 5
mm/min. Testing samples were prepared in dumb-bell shapes and
these dimensions are 100 x
20 x 3 mm3
based on the ASTM D 638 standards. In each case, five samples
were tested and
the average value tabulated.
The thermal characteristics of the epoxy/clay/silica
nanocomposites were measured
using both differential scanning calorimetry (DSC-2010 TA
Instrument) and
76 ILCPA Volume 27
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50 100 150 200 250 300 350 400 450 500 550 600
-4
-2
0
2
4
6
8
10
12
he
at flo
w,W
/g
temperature,°C
0% MC
1% MC
3% MC
10% MC
thermogravimetric analyses (TGA) at a rate of 10 °C/min under
nitrogen flow. The FTIR
spectra of the powders of the untreated and alkali treated
fabric samples were run on an ABB-
Bomem FLATA-2000 model spectrophotometer using KBr pellets. The
concentration of the
fabric powder was maintained at 1 % in KBr.
3. RESULTS AND DISCUSSIONS
3. 1. Curing characteristics
The DSC curing charateristics of epoxy and epoxy filled with MC
are shown in Fig. 1.
It is seen that the addition of MC does not shift the exothermic
peak. The result suggests that
addition of MC fillers in epoxy resin does not affect the curing
of epoxy. The addition of MC
fillers in the epoxy decreases the intensity of exothermic peak.
The clay addition continuously
decreases the exothermic peak and this is due to decrease in
concentration of epoxy resin on
clay addition. It is stated that the presence of organoions in
MC increases the polymerization
of epoxy by catalytic effect [21] and has to increase the curing
temperature. However, the
existence of nanolayers affects the polymerization of epoxy.
Results show that the effect of
nanolayers restricting the polymerization is more than the
polymerization of organoions with
epoxy polymer, and hence decreases the curing temperature of
epoxy resin.
Fig. 1. DSC scans of epoxy with MC series.
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50 100 150 200 250 300 350 400 450 500 550 600
-10
0
10
20
30
40
50
60
70
80
90
100
110
we
igh
t lo
ss,%
temperature,°C
3% MC
0% MC
10% MC
1% MC
3. 2. Thermogravimetry analysis [TGA]
Figure 2 shows decomposition of epoxy filled MC particles. The
MC shows negligible
decomposition up to 600 °C. The addition of MC particles does
not improve the
decomposition of epoxy polymer. There is a marginal shift in
decomposition temperature
when MC is added in to the epoxy polymer. The improved thermal
stability is noticed for
epoxy filled MC series at higher temperatures (>400 °C). The
reinforcement of MC increases
the decomposition of epoxy. The MC addition does not show any
improvement in
decomposition of epoxy polymer.
Fig. 2. TGA thermograms of epoxy with MC series.
3. 3. Tensile properties
Figure 3 shows the effect of clay addition on tensile strength
and tensile modulus. The
tensile strength of pure epoxy is 61.1 MPa. The addition of MC
decreases the tensile strength
of epoxy material. It is seen from Fig. 3 that addition of clay
decreases the strain at break. The
low strain value is due to the formation of voids,
agglomeration, etc. The effect of MC
addition on tensile modulus is seen in Fig. 3. Tensile modulus
of pure epoxy is 3 GPa. On
addition of MC, tensile modulus increases. It is observed that
modulus of nanocomposites
increases continuously with increasing MC content. An
improvement in modulus of ~1.3
times is observed for the addition of 10 % MC. The orientation
of clay platelets and polymer
chains with respect to loading direction can also contribute to
reinforcement effects. The
decreasing rate of modulus at higher clay content (>2 % MC)
is due to presence of
unexfoliated aggregates in epoxy polymer matrix. In epoxy/MC
composites, there is not much
78 ILCPA Volume 27
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improvement in modulus due to the absence of clay distribution
at molecular level, and hence
does not contribute to molecular strength.
0 2 4 6 8 10
50
52
54
56
58
60
62
Tensile strength,MPa
Elastic modulus,GPa
Modified clay content,wt.%
Te
nsile
str
en
gth
,MP
a
2
4
6
8
Ela
stic
mo
du
lud
,GP
a
Fig. 3. Effect of clay content on tensile strength and elastic
modulus.
The tensile fracture surfaces of epoxy and epoxy filled MC are
shown in Fig. 4. If it
seen from Fig. 4 that fracture surface of pure epoxy polymer is
smooth due to brittle failure.
However, on addition of MC particles, crack surface becomes
rough (i.e. ductile nature). The
roughness increases as MC content increases in the matrix. The
fracture roughness indicates
that the resistance of propagation of crack is large and the
crack has not propagated as easily
as seen in pure epoxy.
The fracture surface roughness indicates that crack propagation
is large and increased
the torturous path of propagating crack [24]. This effect
results in higher stress to failure and
caused improved strength of nanocomposites. Though the fracture
roughness is predominant
at 10 % MC, the existence of unexfolaited aggregates, voids,
etc. could have decreased the
strength of nanocomposites. Fracture surface of epoxy with 1 %
MC is rougher than pure
epoxy. At 3 % MC, the presence of voids is noted.
This indicates that particles have peeled off from material as
crack propagates, and
create void at the positions where MC particles were there. This
also indicates that bonding
between matrix and MC particle is poor. For higher clay content
(10 % MC), though the
fracture surface is rough, the existence of voids is clearly
visible and has decreased the
strength of the material.
The poor bonding strength, smooth fracture surface, voids, etc.
could decrease the
tensile strength of the MC filled epoxy composites. It requires
further investigation of the
synthetic procedure to understand the methods of improving
tensile strength for higher MC
contents.
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Fig. 4. SEM tensile fracture surface of (a) E + 1% MC, (b) E + 3
% MC and (c) E + 10 % MC.
3. 4. Impact properties
Impact results of MC filled in epoxy polymer is shown in Fig. 5.
The addition of MC in
epoxy decreases the impact strength of pure epoxy polymer beyond
4 % MC. The decrease in
impact strength at higher filler content is due to the existence
of agglomeration, unexfoliated
aggregates, voids, etc.
The impact fracture surface provides the reason for impact
properties in
nanocomposites.
The existence of rough surface shows that crack propagation is
difficult and could have
increased the torturous path and leads to high strength to
failure.
This has caused high impact strength of silica nanocomposites up
to the addition of 4
wt. % MC, and on higher addition. Impact results suggest that
some additional energy
absorbing mechanism is taking place when nano-particles are
reinforced in matrix.
Crack pinning, cavitation mechanisms, crack surface roughness,
etc. [25] are the
possible reasons for high impact strength of MC filled epoxy
polymer.
80 ILCPA Volume 27
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0 2 4 6 8 10
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9Im
pa
ct str
en
gth
,J/c
m
Midified clay content,wt.%
E+MC series
Fig. 5. Impact strength of epoxy filled with MC series.
3. 5. Vibration characteristics
Table 1 shows the effect of MC addition on natural frequency of
epoxy polymer. The
MC addition does not improve the natural frequency of epoxy as
is seen in the case of FS
filler additions. A negligible effect in stiffness on
reinforcement of MC in epoxy polymer
matrix causes such low natural frequencies.
Figure 6 shows the effect of MC addition on damping
characteristics of pure epoxy
polymer. Damping factors measured by LDM and IHT methods for 1st
and 4th
mode of
natural frequencies are presented. Damping factors measured for
1st mode of natural
frequencies of epoxy with MC series. It is observed that MC
addition increases the damping
factors of pure epoxy.
Damping factor measured by IHT shows higher values than that of
measured by LDM.
The free load during impact causes increased damping in IHT.
Since no free load is acting in
LDM, and hence damping factor is less than that of IHT.
It is seen that damping factor increases up to 3 wt. % of MC,
and for higher MC
addition in epoxy polymer, damping factor decreases but above
the value of matrix material.
The increased stiffness due to the addition of MC improves
damping factor [24]. Similar
effect in damping is noted for 4th mode of natural frequency of
epoxy filled MC. Though the
addition of MC shows improvement in damping factor.
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Table 1. Frequency dependence of epoxy and epoxy filled MC
series.
(E + wt.% MC)
Natural
Frequency at
mode 1, Hz
Natural
Frequency at
mode 2, Hz
Natural
Frequency at
mode 3, Hz
Natural
Frequency at
mode 4, Hz
E+0 17.66 117.10 194.00 297.00
E+1 17.23 122.07 183.75 291.5
E+2 19.51 124.96 191.75 302.42
E+3 20.40 135.08 204.63 295.43
E+5 16.20 111.04 214.06 294.26
E+6 15.24 110.50 201.75 291.43
E+10 15.23 110.45 201.43 291.23
0 2 4 6 8 10
0.030
0.035
0.040
0.045
0.050
0.055
0.060
0.065
0.070
0.075
0.080
0.085
Da
mp
ing
fa
cto
r
Modified clay(MC) content,wt.%
IHT MC
LDM MC
(a)
Fig. 6. Damping factor for epoxy with MC series at (a) 1st mode
and (b) 2nd mode.
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500 1000 1500 2000 2500 3000 3500 4000 4500
25
30
35
40
45
50
55
60
65
70
tra
nsm
itta
nce
,%
wave length,cm-1
3% MC
0 2 4 6 8 10
0.0060
0.0065
0.0070
0.0075
0.0080
0.0085
0.0090
0.0095
0.0100D
am
pin
g f
acto
r
Modified clay (MC) content,wt.%
IHT MC
LDM MC
(b)
Fig. 6(continue). Damping factor for epoxy with MC series at (a)
1st mode and (b) 2nd mode.
(a) Fig. 7. FTIR spectra analysis of (a) E + 3 % MC and (b) E +
5 % MC.
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500 1000 1500 2000 2500 3000 3500 4000 4500
20
25
30
35
40
45
50
55
60
tra
nsm
itta
nce
,%
wave length,cm-1
5% MC
From (Fig. 7) the IR spectra of epoxy filled with MC for 3 wt. %
and 5 wt. %
respectively and the characteristic absorption band of the FS is
at 1500-1600 cm-1
and the
characteristic absorption bands of C–H stretching are shown at
2950 and 3000 cm-1
.
(b) Fig. 7(continue). FTIR spectra analysis of (a) E + 3 % MC
and (b) E + 5 % MC
4. CONCLUSIONS
Room temperature cured epoxy polymer filled with modified clay
is synthesized by
adding TETA curing agent. The MC addition in epoxy matrix does
not affect the peak
exothermic curing temperature of epoxy resin. Tensile property
of nanocomposites shows
enhanced tensile modulus than that of pure epoxy resin and epoxy
filled MC series. The
addition of MC increases the tensile strength of epoxy polymer
and MC addition decreases
the tensile strength of epoxy polymer. Improved impact strength
is noted for epoxy filled with
MC filled epoxy polymer. Natural frequency of nanocomposites is
higher than that of pure
epoxy polymer. Damping factor is increased for 3 wt. % epoxy/MC
series.
84 ILCPA Volume 27
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( Received 29 January 2014; accepted 04 February 2014 )
86 ILCPA Volume 27